OpenSSH penalty behavior in Debian/trixie #newintrixie
This topic came up at a customer of mine in September 2024, when working on Debian/trixie support. Since then I wanted to blog about it to make people aware of this new OpenSSH feature and behavior. I finally found some spare minutes at Debian’s BSP in Vienna, so here we are. :)
Some of our Q/A jobs failed to run against Debian/trixie, in the debug logs we found:
debug1: kex_exchange_identification: banner line 0: Not allowed at this time
This Not allowed at this time pointed to a new OpenSSH feature. OpenSSH introduced options to penalize undesirable behavior with version 9.8p1, see OpenSSH Release Notes, and also sshd source code.
FTR, on the SSH server side, you’ll see messages like that:
Apr 13 08:57:11 grml sshd-session[2135]: error: maximum authentication attempts exceeded for root from 10.100.15.42 port 55792 ssh2 [preauth] Apr 13 08:57:11 grml sshd-session[2135]: Disconnecting authenticating user root 10.100.15.42 port 55792: Too many authentication failures [preauth] Apr 13 08:57:12 grml sshd-session[2137]: error: maximum authentication attempts exceeded for root from 10.100.15.42 port 55800 ssh2 [preauth] Apr 13 08:57:12 grml sshd-session[2137]: Disconnecting authenticating user root 10.100.15.42 port 55800: Too many authentication failures [preauth] Apr 13 08:57:13 grml sshd-session[2139]: error: maximum authentication attempts exceeded for root from 10.100.15.42 port 55804 ssh2 [preauth] Apr 13 08:57:13 grml sshd-session[2139]: Disconnecting authenticating user root 10.100.15.42 port 55804: Too many authentication failures [preauth] Apr 13 08:57:13 grml sshd-session[2141]: error: maximum authentication attempts exceeded for root from 10.100.15.42 port 55810 ssh2 [preauth] Apr 13 08:57:13 grml sshd-session[2141]: Disconnecting authenticating user root 10.100.15.42 port 55810: Too many authentication failures [preauth] Apr 13 08:57:13 grml sshd[1417]: drop connection #0 from [10.100.15.42]:55818 on [10.100.15.230]:22 penalty: failed authentication Apr 13 08:57:14 grml sshd[1417]: drop connection #0 from [10.100.15.42]:55824 on [10.100.15.230]:22 penalty: failed authentication Apr 13 08:57:14 grml sshd[1417]: drop connection #0 from [10.100.15.42]:55838 on [10.100.15.230]:22 penalty: failed authentication Apr 13 08:57:14 grml sshd[1417]: drop connection #0 from [10.100.15.42]:55854 on [10.100.15.230]:22 penalty: failed authentication
This feature certainly is useful and has its use cases. But if you f.e. run automated checks to ensure that specific logins aren’t working, be careful: you might hit the penalty feature, lock yourself out but also consecutive checks then don’t behave as expected. Your login checks might fail, but only because the penalty behavior kicks in. The login you’re verifying still might be working underneath, but you don’t actually check for it exactly. Furthermore legitimate traffic from systems which accept connections from many users or behind shared IP addresses, like NAT and proxies could be denied.
To disable this new behavior, you can set PerSourcePenalties no in your sshd_config, but there are also further configuration options available, see PerSourcePenalties and PerSourcePenaltyExemptList settings in sshd_config(5) for further details.